The present invention relates to a method for the interior coating of hollow bodies by means of applying a coating medium and with the help of heat which is applied to the body as well as an apparatus to perform this method, said apparatus comprising an arm over which the bodies are moved and whereby there is further provided a coating arrangement on said arm.
Problems which occur for instance with respect to corrosion at joined edges of hollow bodies, joined e.g. by soldering or welding, of bodies as of metal sheet material for the manufacture e.g. of cans are known since long. For the solution of these problems the CH PS No. 444 632 proposes after the edges having been welded, to provide soldering over the welding seam so that the solder, flowing over the welding seam provides for a corrosion protective layer. This patent further proposes to coat additionally the seam with the help of a coating nozzle to further enhance corrosion protection. The metal sheet which is, after having been welded, soldered, is heated up by means of a not specifically defined heat source to enhance proper flow of the solder, a measure which is especially taken, if welding and soldering are separated either locally or with respect to the time at which welding and soldering occurs.
The German Pat. No. 714 669 proposes to resolve the problems mentioned above to coat the yet unjoined edges of the bodies with a lacquer then to solder these edges whereby the lacquer is chosen so that the soldering temperature provides for burning in of the lacquer.
This latter method which makes use of the same energy for joining the edges and for burning in the lacquer may not be used for welding-joint technique of the edges as the welding heat would not burn in the lacquer but would just burn the lacquer and provision of the lacquer would harm proper welding.
The German Pat. No. 750 056 and the European publication No. 0 120 810 disclose methods which provide for the application of a protecting layer from the inner side of the bodies after the edges of the bent bodies have been linked. This interior coating is, according to the European publication No. 0 120 810, afterwards burnt in whereby there is provided a heat source which acts from the exterior side on the finished hollow bodies.
As heating sources, inductive heaters, gas burner heaters or infrared radiators may be used. By the latter mentioned method the complete cylindrical body is brought on the burn in temperature level and is kept on that temperature for a predetermined time to provide for proper burning in. Especially when this method is used on hollow bodies of clean metal there occurs a high degree of heat reflection and the efficiency of the heat source acting from the exterior side on the body is accordingly bad with respect to the burn in target which is to be reached by provision of said heat source.
As concerns efficiency, the same considerations are valid if, for interior powder coating of the bodies to be coated, there is provided preheating from exterior with the object to enhance adhesion of the powder applied to the interior body walls. Here too exterior heating provides for heating of huge parts of the bodies and the heat capacity of the bodies provides for moving a high amount of heat into the region where coating means are provided so that this excessive heat may cause coating nozzles to be heated up as well, so that the coating powder may stick on such nozzles and affect their openings.
It is an object of the present invention at a method as mentioned above to apply the heat used pointed to an area where this heat is in fact necessitated. This object is reached by heating up the bodies before and/or after appliance of the coating directly from the body inner side.
For burning in coating there is thus directly applied burning-in heat to the body from its inner side after provision of the coating.
An apparatus to perform the inventive method comprises at the arm a heat source arrangement situated upstream and/or downstream of the coating arrangement.
By the inventive method as well as the according apparatus, the object is fulfilled that the heat applied is pointed directly on the interior surface of the bodies where it is in fact used.
A further advantage which is reached by the inventive method and the according apparatus is that thermal loading of the region just exterior from the bodies is kept much smaller as with known methods which use exterior heating, a fact which leads, besides of improving efficiency, further to the advantage that there may be provided at least between the coating arrangement and the burning-in heat source arrangement, in the region of coating, a continuous conveyer belt arrangement with at least one conveyer belt to move the bodies from the coating arrangement to the burn in heat source arrangement.
If heat is applied from the exterior of the bodies then a conveyer belt arrangement which continuously moves the body between the coating arrangement and the burn in heat source arrangement must be mounted on the opposite side of the bodies with respect to the heat source as is shown in the European publication No. 0 120 810 to prevent the conveyer belts to be too heavily thermically loaded by the heat sources or there must be taken as conveyer belt material expensive heat resistent material.
Because of the inventively pointed heat appliance the conveyer belt arrangement may be mounted in the coating region that is acting on the bodies in their seam region so that the degree of freedom for constructing an overall apparatus comprising e.g. a welding station, a coating station, a burn in station and conveyer means to convey the bodies from one to the other station, becomes considerably larger.
Preferably the heat source arrangement comprises at least one infrared radiator.
By providing a temperature measurement at the region of the arm, monitoring the temperature status during treatment of the bodies becomes possible.
Thereby preferably the result of said temperature measurement, a signal according to the momentarily temperature status, as a controlled output signal, is applied to an input for a controlled value of a temperature feedback control loop, which latter further comprises said heat source arrangement as adjusting member, so that there may be applied to the temperature feedback control loop a desired temperature value, as a control signal by means of an adjustable control signal generator unit and the controlled temperature value according to the temperature measurement is feedback controlled to stay on the temperature control value applied.
To further improve pointed heat appliance it is a further object of this invention to provide at said arm at least one reflector arrangement which concentrates the heat radiation of the heat source arrangement to a predetermined region or area of the bodies.
Preferably the heat source arrangement is arranged axially along a part of the arm and radially separated therefrom whereby said reflector arrangement comprises a reflecting groove arranged with respect to the source arrangement towards said arm, whereby said reflecting groove is preferably worked into the arm and is provided with a reflecting surface.
Considering the fact that can production as production of said hollow bodies occurs nowadays with speeds up to 650 cans/min., it becomes obvious that a reduction of the heat amount necessitated for the interior can coating to reach well defined results leads to tremendous savings with respect to energy, i.e. in both cases, either if the heat is applied to improve adhesion of a coating medium on the body walls or if the heat is applied to burn in a coating on the interior body side or if the heat is applied for both targets. The inventively pointed heat application and especially with said reflector arrangement provided, reduces heating up of the arm drastically so that additional problems of construction are resolved which occur by the fact that all feeding leads and pipes, as well for the coating arrangement as for the heat sources provided, e.g. electrical leads, run along the arm or within the arm whereby said pointed heat application prevents too high thermical loading of the arm and thus said leads are also uncritically loaded by heat.
Examples of preferred embodiments of the invention will now be described with the help of Figures.